1. Field of the Invention
This invention relates to an operational amplifier, especially to a high slew rate operational amplifier used for analog data transmission in a LCD panel and operating method thereof.
2. Description of the Related Art
With the progress of liquid crystal display technology, panel size and pixel number of the LCD panel are also increased. In order to meet the requirement of the large-size and high-resolution LCD apparatus, a slew rate of an operational amplifier used for analog data transmission in the LCD panel should be also enhanced, so that the user will not see different data transmission rates in different pixels on the LCD panel.
The definition of the so-called “slew rate” is the increased amplitude of voltage in 1 microsecond (ms). Its unit can be V/s, V/ms, or V/μs. As to the operational amplifier, the slew rate is an important parameter to measure the speed of the operational amplifier. The slew rate of the operational amplifier means a capability of the operational amplifier to follow or respond burst signals or pulse signals, namely a transient response capability. If the slew rate of the operational amplifier is slow, a condition of transient inter-modulation distortion will become more serious.
In general, there are two types of conventional high slew rate operational amplifiers: the first type is to directly increase the current source of the operational amplifier, but the static power consumption of entire circuit will be also increased; another type is to use a slew rate enhancement (SRE) circuit.
Please refer to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 illustrate different types of SRE circuits used in conventional operational amplifier circuits respectively. The conventional operational amplifiers use a first SRE circuit SRE1 of FIG. 1 and a second SRE circuit SRE2 of FIG. 2 to sense input terminals INP and output terminals OUT of the operational amplifier circuits to provide appropriate slew rate compensation to the operational amplifier circuits.
However, since the first SRE circuit SRE1 and the second SRE circuit SRE2 use the threshold voltage of transistor to determine whether to provide slew rate compensation or not, only when the voltage difference between the input terminal INP and output terminal OUT is larger than the threshold voltage of transistor, the first SRE circuit SRE1 and the second SRE circuit SRE2 will be triggered to provide appropriate slew rate compensation to the operational amplifier circuits. This will make the threshold voltages of the first SRE circuit SRE1 and the second SRE circuit SRE2 less flexible to be adjusted. When the change of the data in the operational amplifier circuits is small, the first SRE circuit SRE1 and the second SRE circuit SRE2 would be triggered and then shut down immediately, so that the slew rate of the operational amplifier circuit can be effectively increased.
In addition, there is another drawback of the conventional operational amplifier circuit: if a bulk electrode (a body electrode) and a source electrode of the transistor M1 or M1′ comparing the voltages of the input terminal INP and the output terminal OUT fail to be connected, the transistor M1 or M1′ will generate a body effect. Therefore, the threshold voltage of the transistor M1 or M1′ will be varied with the voltage change of the source electrode, and the threshold voltage used to trigger the SRE circuit will be also changed. Because the threshold voltage used to trigger the SRE circuit is changed, it is hard to provide appropriate slew rate compensation to the operational amplifier circuit. And, the conventional first SRE circuit SRE1 and second SRE circuit SRE2 only use one stage slew rate enhancement to drive the operational amplifier circuit, its slew rate compensation speed is too slow to meet nowadays requirements.